A water-repellent film, which can repel water and oil and allow easy removal of materials attached to the surface thereof, has been used widely in various fields. For example, by forming a water-repellent film on windows of an automobile, it is possible to secure an excellent view because the windows can repel water even on rainy days.
Furthermore, by forming a water-repellent film on such places as the surface of cooking equipment, kitchen, bathroom, and the like, dirt can be removed easily from such places, and consequently, the care thereof becomes easy. Furthermore, in recent years, such a water-repellent film has been used as a main component of an ink jet nozzle making up an ink jet head of an ink jet type recording apparatus. The reason why the ink jet nozzle requires a water-repellent film will be described below in detail.
The mechanism of printing by the ink jet type recording apparatus is to discharge several tens pico-liter of ink from each of a large number of nozzle holes with a diameter of several tens μm, which are bored in a nozzle plate, onto a printed medium such as paper so as to arrange the discharged ink at a predetermined position on the printed medium. In order to arrange the ink at a predetermined position on the printed medium, the ink is discharged while mechanically moving the nozzle plate and the printed medium respectively to control their relative position. FIG. 8A is a cross-sectional view showing a nozzle hole 34 and its vicinity, where an ink space 31 for storing a fixed amount of ink 32 is formed on an inner surface of a nozzle plate 33 having a through hole from which the ink 32 is discharged. As shown in FIG. 8B, the ink space 31 is designed so that the pressure in the space can be increased as needed, for example, by means of mechanical deformation of a piezoelectric thin film 35. By increasing the pressure in the ink space 31, a predetermined amount of ink 36 can be discharged through the through hole of the nozzle plate from the ink space as indicated by an arrow 37. Here, in order to conduct high-definition printing, the ink discharged from the through hole should be arranged on the printed medium accurately. To this end, it is necessary to control precisely the relative position of the nozzle and the printed medium, to control the amount of discharged ink and make the same minute, and to control precisely the discharging direction of the ink. Among these, in order to control precisely the discharging direction of the ink, the discharging direction of the ink should be perpendicular to the nozzle plate face. Here, as shown in FIG. 9A, if ink 45 remains at a portion of the periphery of the hole, the discharging direction of ink 47 will lean toward the remaining ink side and deviate from the perpendicular direction as shown by an arrow 48 of FIG. 9B. Because the attractive force occurs due to the surface tension between the ink 47 and the remaining ink 45, the ink 47 from the nozzle 34 is attracted toward the side of the ink 45. To avoid this situation, in normal printers, remaining ink is removed by wiping the periphery of holes regularly with a rubber blade. Here, in order to clearly remove the remaining ink by wiping it with the rubber blade, it has been found that an ink-discharging plane of a nozzle plate should have water repellency. For that reason, on a surface of an ink jet nozzle plate, various water-repellent films are formed.
Conventionally, in order to form a water-repellent film on a solid substrate, in general, polytetrafluoroethylene (PTFE) and the derivatives thereof, which have water repellency, have been applied to the substrate to form a film. However, PTFE and the derivatives thereof have a small surface energy and even if they are applied directly to the substrate to form a film, the film peels off the substrate easily. Therefore, in order to secure the adhesion between the film and the substrate, there have been employed a method of roughening the surface of the substrate and then applying a water-repellent film to the roughened surface, and a method of roughening the surface of the substrate, and forming a primer layer (adhesive layer) made of polyethylene sulphide, etc., on the roughened surface, followed by sintering a water-repellent film. Furthermore, when the solid substrate is made of a metal, a method of plating particles of PTFE and the derivatives thereof together with the metal may be employed.
On the other hand, there have been proposed methods of forming a water-repellent film having an excellent adhesion directly on the surface of a substrate by using a silane coupling agent without roughening the surface of the substrate. As the first example of these methods, there is described a method for forming a water-repellent monomolecular or polymer film by allowing fluoroalkyl trichlorosilane such as CF3(CF2)8C2H4SiCl3 to react with a substrate (JP 2500816 and JP 2525536). In the above-mentioned chemical formula, CF3(CF2)8C2H4— represents a fluoroalkyl group, and —SiCl3 represents a trichlorosilyl group. In this method, a substrate having active hydrogen on the surface thereof is brought into contact with a solution in which fluoroalkyl trichlorosilane is dissolved so as to allow a chlorosilyl group (—SiCl) to react with active hydrogen, thus forming —Si—O— bonding to the substrate. As a result, a fluoroalkyl chain is fixed to the substrate via —Si—O—. Herein, the fluoroalkyl chain provides a film with water repellency. Depending on the film formation conditions, the water-repellent film becomes a monomolecular film or a polymeric film. As the second example, there is described a method in which a porous substrate impregnated with a compound containing fluorine such as fluoroalkyl alkoxysilane is heated in a vacuum to evaporate the compound, thus providing the surface of the substrate with water repellency (JP 6 (1994)-143586A). In order to improve the adhesion between the water-repellent film and the substrate, this method proposes that an intermediate layer made of silicon dioxide, etc. is provided. As the third example, there is described a method of forming a film made of titanium, titanium oxide or indium-tin oxide on the substrate, and forming a fluoroalkyl silane based compound thereon by a vacuum evaporation method (JP 10 (1998)-323979A). As the fourth example, there is described a method of forming fine particles of oxides such as zirconia and alumina on the surface of a substrate, and then applying a fluorine based silane coupling agent such as fluoroalkyl chlorosilane and fluoroalkyl alkoxysilane (JP 6 (1994)-171094A). As the fifth example, there is described a method of subjecting a mixed solution, which is obtained by adding metal alkoxide to fluoroalkylalkoxysilane such as CF3(CF2)8C2H4Si(OCH3)3, to hydrolysis and dehydration polymerization, and then applying the solution to the substrate, followed by baking, thereby forming a water-repellent film in which molecules having a fluoroalkyl chain are mixed in the metal oxide (JP 2687060, JP 2874391, JP 2729714 and JP 2555797). In these methods, a fluoroalkyl chain provides the film with water repellency, and a metal oxide provides the film with a high mechanical strength.
The water repellent film has to be produced by selecting an optimum method among various formation methods as described above depending on its intended use. For the formation of a water-repellent film used for an ink jet nozzle, especially, a method utilizing a silane coupling agent is more effective than the other methods because of the following respects. Firstly, a water-repellent treatment can be conducted basically on any substrate of a nozzle. Secondly, a water-repellent film can be made thin. The reason for requiring the thin film is, as shown in FIG. 10A, that side faces of a nozzle through hole 34 that is bored in a water-repellent film 55 should have water repellency and have a small film thickness. A large film thickness of a water-repellent film 56 as shown in FIG. 10B makes it impossible for ink 32 to be discharged from an ink space 31 through a through hole 34 due to the water repellency of the side faces thereof. To avoid this situation, the film thickness should be made sufficiently smaller than a nozzle diameter. For this reason, it can be considered that the second feature will increasingly become important. This is because the tendency for requiring high-definition printing will be increased in the future, and accordingly a nozzle diameter will be reduced, and therefore a water-repellent film with a film thickness smaller than such a nozzle diameter will be demanded. Therefore, the above-mentioned second to fourth methods are employed for forming a water-repellent film used for an ink jet nozzle.
Since a water-repellent film using a silane coupling agent can be formed on various substrates without performing a pretreatment, it can be expected to be applied in various fields. It is particularly useful in an ink jet head application. However, a conventional water-repellent film using a silane coupling agent lacks durability against alkalinity. Especially, in the application to an ink jet head, this is a serious problem. This is because the ink used for an ink jet type recording apparatus generally is alkaline, and therefore a water repellent-film used for the ink jet head is required to have durability against an alkaline solution.
The conventional monomolecular film or polymer film using a silane coupling agent is bonded to the substrate via Si—O bonding. However, since this bonding is hydrolyzed easily in an alkaline solution, when the conventional water-repellent film is soaked in an alkaline solution, the bonding disappears easily from the substrate. That is, such a film lacks durability against an alkaline solution.
Then, the third and fourth examples provide a method in which in order to improve the alkali resistance, an alkali resistant lower film made of titanium oxide, titanium, zirconia particles, alumina particles, etc. is formed under a water-repellent film. Thus, a water-repellent film hardly is peeled off the solid substrate due to the lower layer breaking away. On the other hand, even with this configuration, the problem that hydrogen bonding or siloxane bonding between the water-repellent film and the substrate is broken by alkaline has not been solved completely.
The water-repellent films proposed in the conventional methods use a silane coupling agent having a reactive group only at one end of a straight-chain molecule, for example, fluoroalkyl alkoxysilane and fluoroalkyl chlorosilane, etc. In such a coupling agent, as shown in FIG. 11, due to the steric hindrance between molecules, three-dimensional polymerization between molecules hardly occurs and the film density is lower than that of a general polymeric polymer. A silane coupling agent 61 causes a hydration reaction 62 with a hydroxyl group on the surface of the substrate to form siloxane bonding, or is fixed by hydrogen bonding. Therefore, as shown in FIG. 12A, as the substrate 71 has the higher density of hydroxyl groups on the surface of the substrate, the density of the film (a film of the silane coupling agent bonded to the substrate) 72 in the vicinity of the substrate becomes higher. Herein, as shown in FIG. 12B, in the case of a lower film 73 made of titanium oxide, titanium, zirconia, etc., since the density of hydroxyl groups on the surface of such a film is low, the density of the water-repellent film (a film of the silane coupling agent bonded to the substrate) 74 that is in contact with the lower film is low. FIG. 13 is a schematic view showing a state in which water-repellent films 82 and 81 formed on a lower layer 83 having a low density of hydroxyl groups are exposed to an alkaline component. In the nearer part of the lower film 83, the silane coupling molecules (a water-repellent film in the vicinity of the lower film) 82 are fixed to the lower film 83 via hydrogen bonding and siloxane bonding, and in the more distant part from the lower film 83, the low density water-repellent film (a water-repellent film distant from the lower film) 81 is formed. When an alkali ink is brought into contact with this film, hydroxyl ions (OH−) 84 as alkaline components pass through the film 81 and go to the lower film 83. When the density of the water-repellent film 82 in the vicinity of the lower film is small, ions 85 enter into the interface between the film 82 and the lower film 83 and break the hydrogen bonding and the siloxane bonding present therein. Even if the lower film has much durability against an alkaline solution, if the density of hydroxyl groups on the surface thereof is low, the alkali resistance of the water-repellent film decreases.
Furthermore, in order to improve the alkali resistance, the fifth example is useful, in which a molecule having a fluoroalkyl chain is mixed in the metal oxide such as titanium oxide and zirconium oxide, which has the durability against an alkaline solution. However, these metal oxides have to be produced by subjecting titanalkoxide and zirconiumalkoxide to hydrolysis and dehydration polymerization, and these alkoxides have high reactivity and hydrolysis proceeds quickly in the air. Therefore, it is hard to handle a coating solution using these alkoxides for applying a water-repellent film. Therefore, siliconalkoxide that is stable in the air has been used widely. However, silicon oxide formed from the siliconalkoxide is solved in an alkaline solution. Therefore, there is a problem that the water-repellent film using siliconalkoxide has low durability against an alkaline solution.